I. Field of the Invention
The invention relates generally to wireless communications. More particularly, the invention relates to environmental verification by test, demonstration or analysis of a wireless communication device.
II. Description of the Related Art
FIG. 1 is an exemplifying embodiment of a terrestrial wireless communication system 10. FIG. 1 shows the three remote units 12A, 12B and 12C and two base stations 14. In reality, typical wireless communication systems may have many more remotes units and base stations. In FIG. 1, the remote unit 12A is shown as a mobile telephone unit installed in a car. FIG. 1 also shows a portable computer remote unit 12B and the fixed location remote unit 12C such as might be found in a wireless local loop or meter reading system. In the most general embodiment, remote units may be any type of communication unit. For example, the remote units can be hand-held personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link signal 18 from the base stations 14 to the remote units 12 and a reverse link signal 20 from the remote units 12 to the base stations 14. Such systems typically offer voice and data services. Other modern communication systems operate over wireless satellite links rather than through terrestrial base stations.
In a code division multiple access (CDMA) system, remote units use a common frequency band for communication with all base stations in the system. Use of a common frequency band adds flexibility and provides many advantages to the system. For example, the use of a common frequency band enables a remote unit to simultaneously receive communications from more than one base station as well as transmit a signal for reception by more than one base station. The remote unit can discriminate and separately receive the simultaneously received signals from the various base stations through the use of the spread spectrum CDMA waveform properties. Likewise, the base station can discriminate and separately receive signals from a plurality of remote units. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein. An industry standard for a wireless system using code division multiple access (CDMA) is set forth in the TIA/EIA Interim Standard entitled "Mobile Station Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", TIA/EIA/IS-95, and its progeny (collectively referred to here in as IS-95), the contents of which are also incorporated herein by reference.
CDMA communication techniques offer many advantages over narrow band modulation techniques. In particular, the terrestrial channel poses special problems due to the generation of multipath signals which can be overcome through the use of CDMA techniques. For example, at the base station receiver, separate multipath signals from a common remote unit signal can be discriminated and separately received using similar CDMA techniques as those used to discriminate between signals from the various remote units.
In the terrestrial channel, multipath signals are created by reflection of signals from obstacles in the environment, such as trees, buildings, cars and people. In general, the terrestrial channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over a multipath channel, a stream of pulses is received. In a time varying multipath channel, the received stream of pulses changes in time location, amplitude and phase as a function of the time at which the ideal impulse is transmitted.
Because the response of the remote unit to multipath is a significant factor in determining the overall performance of the remote unit, it is important to test this response--both statistically and on a unit-by-unit basis. One way multipath testing has been executed in the past is by creation of artificial multipath signals. See, for example, U.S. Pat. No. 3,761,825, entitled "MULTIPATH SIMULATOR FOR MODULATED R.F. CARRIER SIGNALS." However, such techniques develop the multipath signals on a wired connection which is designed to be directly coupled to the device under test. Such direct connections bypass the antenna structure and, therefore, do not provide a complete assessment of the response of the device under test to wireless multipath received through the antenna.
Another way in which such tests have been performed in the past is to place the remote unit in a test vehicle and collect test data as the vehicle moves around within a real world environment. The disadvantage of such tests is that they are not highly repeatable and a significant amount of data must be collected on each device under test to be statistically significant. For example, on one test run, the device under test may be subjected to extreme multipath if it comes to rest beside a large semi-truck at a stop light. On another run, traffic may be very light, thus, generating less multipath. The data collected from each of these runs might be vastly different even though the two devices have the same performance because the stimulating signal is not deterministic or controllable.
In addition, the real world environment imposes other significant effects aside from multipath. For example, with hand-held units, the hand and head of the user affect the far field antenna characteristics. Extensive work has been done to characterize these effects. See, for example, Technical Papers from the Colloquium on DESIGN OF MOBILE HANDSET ANTENNAS FOR OPTIMAL PERFORMANCE IN THE PRESENCE OF BIOLOGICAL TISSUE at Savory Place on Monday Jan. 20, 1997, organized by Professional Group E11 (Antennas and Propagation) and co-sponsored by Professional Groups E8 (Radiocommunication Systems) and S9 (Biomedical Engineering) of the Institute of Electrical Engineers, Savory Place, London. In this way, if real-world tests are run, the results may depend upon the physical characteristics of the technician executing the tests, thereby further decoupling the results of the test from the actual performance of the device under test.
In order to overcome the problems with real world testing, simulated environments have been developed to test the performance of wireless devices. See, for example, U.S. Pat. No. 4,106,345 entitled "SIMULATION FACILITIES FOR DYNAMICALLY TESTING GUIDANCE SYSTEMS WHICH USE RADIOMETRIC FREQUENCIES" However, such environments do not perform the functions necessary to test a wireless remote unit whose performance is highly dependent upon the statistical nature of the channel environment. Thus, there has been a need in the art for a means for and method of simulating an environment for a wireless communication device which is easy to control and provides repeatable results.